Stable carbon isotope composition of atmospheric methyl bromide

Methyl bromide (CH3Br) constitutes the largest source of bromine atoms to the stratosphere and plays an important role in stratospheric ozone depletion. However, the CH3Br atmospheric budget remains uncertain. We report the first measurements of stable carbon isotope ratios for atmospheric CH3Br and explore the constraints they provide for the atmospheric budget. The measured delta(13)C of CH3Br ranged from - 44.8parts per thousand to - 41.2parts per thousand ( mean value - 43.1parts per thousand, N = 7). This isotopic value is less negative than the anthropogenic source signature ( - 53.2 to - 47.5parts per thousand), but is indistinguishable from the only quantified natural source signature ( - 43.0parts per thousand). Based on known source signatures and our atmospheric measurements, we predict that the mean kinetic isotope effect associated with sinks must be small ( 2 - 4parts per thousand). We further estimate that a total phaseout of anthropogenic sources would likely change the isotopic composition of atmospheric CH3Br by 2 - 4parts per thousand, and that this change would be difficult to detect with current measurement techniques.